1. Field of the Invention
This invention relates to devices requiring soldering of a component to a substrate and, in particular, devices soldered using a solder paste.
2. Art Background
Many electronic devices are formed by soldering components onto a substrate. Exemplary of such devices arc printed circuit boards, multichip modules, and hybrid integrated circuits. (See R. R. Tummala et al, Micoelectronics Packaging Handbook, pp. 366-391 and 819-836, Van Nostrand Reinhold, New York, N.Y. (1989), for a review of typical devices formed using soldering techniques and the soldering techniques employed.) In each device, electrical conducting, e.g., metallic, interconnect lines are formed on a substrate and used to provide the required electrical interconnections between component leads.
A variety of ways are available to apply the solder onto the metallic interconnect line where the component is to be soldered. For example, one conventional approach involves soldering to a component whose leads are deposed through a hole in the circuit board. A second procedure involves surface mounting of components. In leadless surface mounting, metal regions on the bottom of the component are aligned and soldered to corresponding pads on the metallized interconnect lines. Similarly, in leaded surface mounting, metal tabs on the components are soldered to corresponding pads on the interconnect. Another approach is commonly denominated flip-chip technology and involves positioning metallized regions on unpackaged component and corresponding pads on the metallized interconnects that are to be electrically connected to the corresponding pads on the component. In some flip-chip applications, the metallized regions on the component and on the interconnects are prebumped with solder prior to the assembly.
Irrespective of soldering approach, the solder applied between the component and the interconnect substrate typically includes both a vehicle and dispersed solder--typically particles. (A solder material is considered a metallic alloy with melting point lower than 35.degree. C., and a vehicle is considered a medium that maintains the particles of the solder material in a cohesive mass; allows their dispension; and promotes the fusion of the solder particles when heated to the melting point.) The combination of the vehicle and solder particles into a dispersion yields a paste. The paste is applied to the component and to the substrate interconnects in a variety of ways. One particularly advantageous approach involves the application of the paste through a stencil with openings corresponding to the regions upon which paste is desired. This approach is advantageous because it is relatively inexpensive and is expeditiously performed. Other approaches for applying the paste include pneumatic dispensing; or pin transfer, as disclosed in J. S. Hwang, Solder Past in Electronic Packaging, pp. 172-174 and 180-181, Van Nostrand Reinhold, New York, N.Y. (1989).
Even if the solder paste is applied precisely to the interconnect regions where solder is desired, problems are still possible. For example, when the devices are placed on the paste, the solder paste can spread from one interconnect metallic line to an adjacent line across the intervening dielectric substrate portion. Upon reflow, this may lead to a solder "bridge" between these lines. This intrusion of solder from one interconnect line to a second produces a short circuit which is unacceptable. Alternatively, the spread solder paste may segregate upon reflow between adjacent interconnect lines and produces one solder connection with an excess of solder and one starved of solder. The latter condition often presents reliability problems. Similarly, solder paste on pads close to the edge of the component can be extruded from under the component (FIG. 1 left side paste, 3, extruded from under component 2) and thus yield, upon reflow, a solder starved joint and solder debris both of which are considered reliability problems.
The most common approach for solving this difficulty has generally been to space interconnect lines sufficiently far apart that such short circuits do not occur. However, the objective in device fabrication has long been to increase the density of the components electrically interconnected on a substrate. Therefore, an increase in a metallic line spacing is not a desirable solution to the difficulties associated with soldering on fine line devices.